Re: [PATCH 1/3] x86: Reject bad %dr6/%dr7 values when loading guest state

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/08/2023 7:46 am, Jan Beulich wrote:
> On 29.08.2023 15:43, Andrew Cooper wrote:
>> --- a/xen/arch/x86/domain.c
>> +++ b/xen/arch/x86/domain.c
>> @@ -1074,8 +1074,27 @@ int arch_set_info_guest(
>>  #endif
>>      flags = c(flags);
>>  
>> +    if ( !compat )
>> +    {
>> +        if ( c(debugreg[6]) != (uint32_t)c(debugreg[6]) ||
>> +             c(debugreg[7]) != (uint32_t)c(debugreg[7]) )
>> +            return -EINVAL;
>> +    }
>> +
>>      if ( is_pv_domain(d) )
>>      {
>> +        /*
>> +         * Prior to Xen 4.11, dr5 was used to hold the emulated-only
>> +         * subset of dr7, and dr4 was unused.
>> +         *
>> +         * In Xen 4.11 and later, dr4/5 are written as zero, ignored for
>> +         * backwards compatibility, and dr7 emulation is handled
>> +         * internally.
>> +         */
>> +        for ( i = 0; i < ARRAY_SIZE(v->arch.dr); i++ )
>> +            if ( !access_ok(c(debugreg[i]), sizeof(long)) )
>> +                return -EINVAL;
>> +
>>          if ( !compat )
>>          {
>>              if ( !is_canonical_address(c.nat->user_regs.rip) ||
> One more thing here: v->arch.dr is an array of 4 elements, i.e. doesn't
> cover %dr4 and up.

Correct (as of the same changeset relevant in this comment).

> That's not directly visible here, though, so the
> comment ahead of the loop talking about those other 4 registers is a
> little misleading. Would you mind moving it below the loop?

Can do.

~Andrew